Nile perch population explosion studied

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The colonisation of Lake Victoria by Nile perch (Lates niloticus) and the consequent decline of the haplochromine fishes in the lake has been elucidated in a study published in the most recent issue of the journal Environmental Biology of Fishes.

In the paper by Kees Goudswaard, Frans Witte and Egid Katunzi, the process by which the Nile perch has colonized Lake Victoria and its effects on the haplochromine fauna of the lake was studied using catch data from numerous surveys.

Although Nile perch were introduced unofficially into the Ugandan portion of the lake in 1954 (with official introductions being made in 1962"63), the populations remained low until surges occurred in 1980s (although the surges did not occur simultaneously in parts of the lake where the Nile perch was present).

These surges were thought to have been responsible for the decline and extinction of several hundred species of haplochromine cichlids in the lake, but the study suggests that the decline was initiated by the onset of trawl fisheries for haplochromines, which began in the 1970s.

The role of haplochromine cichlids in keeping the population of the Nile perch low is explained by the authors: It is remarkable that after the first release of Nile perch in Lake Victoria it took more than 25 years before a sudden increase of its population took place, resulting in its dominance in the demersal fish stock.

The initial abundance of haplochromine cichlids suggests that food cannot have been a limiting factor for the piscivorous Nile perch between its introduction and the early 1980s.

Therefore, we assume that other factors have played a role. An important factor could have been the survival of very young Nile perch and the food abundance for these fishes.

Although we have no evidence from stomach contents, it is likely that the abundant haplochromines fed upon eggs and larvae of Nile perch, whenever available. Moreover, the haplochromines were probably competing with juvenile Nile perch for zooplankton and insect larvae.

The colonisation process the adult and sub-adult Nile perch fed heavily upon haplochromine cichlids...and reduced their numbers to extremely low levels, thus enhancing the chance of survival of eggs and larvae of Nile perch.

According to the authors, the following is the most plausible scenario for the colonization of Nile perch in Lake Victoria: (1) Initially, over-fishing of haplochromines in the Nyanza Gulf facilitated a local recruitment of juvenile Nile perch due to a decrease of predation by and competition with haplochromines. (2) Colonization of other areas in the lake took place at different moments. In each of these areas the first step in the process comprised the invasion of sub-adults from neighbouring areas, where haplochromines had been depleted. (3) Subsequently, the local haplochromine stock vanished, and both juvenile Nile perch and juvenile shrimps had a higher survival rate than before. (4) As a consequence a strong recruitment of Nile perch occurred in the area.

For more information, see the paper: Goudswaard, PC, F Witte and EFB Katunzi (2008) The invasion of an introduced predator, Nile perch (Lates niloticus, L.) in Lake Victoria (East Africa): chronology and causes. Environmental Biology of Fishes 81, pp. 127"139.